The present invention generally relates to ink compositions and delivery systems associated therewith, and more particularly to invisible ink materials which include one or more chemical agents therein that cannot be seen by the unaided eye but will fluoresce when exposed to (1) far red ("FR") light; (2) infrared ("IR") light; or (2) ultraviolet ("UV") light, depending on the particular chemical agents being used. The resulting fluorescent emission can then be detected through the use of a suitable detection system when far red/infrared light is applied or observed by the unaided eye when ultraviolet illumination is employed. The invisible ink compositions have many uses in a variety of different fields and represent an advance in invisible imaging technology.
In recent years, the demand for effective "invisible" ink compositions has steadily increased. Invisible ink materials are traditionally defined to involve a broad class of ink formulations which cannot be seen by the unaided eye when applied to a substrate and viewed with "natural" light (e.g. light from the sun) or light from conventional incandescent lamps and the like. Both of these light forms (as well as other forms which are normally used for general illumination purposes in homes, businesses, and the like) are collectively characterized as "white" light which involves a combination of all the various colored light fractions which fall within a wavelength range of about 300-700 nm. Under these illumination conditions, the ink compositions are essentially colorless. Only after illumination with other, more narrow light wavelengths do the printed images become visible or otherwise detectible (either with or without auxiliary observation equipment).
The uses of these materials are varied and widespread. For example, invisible ink products provide many benefits when printed on a variety of documents including insurance policies, checks, and other related materials. Of particular interest is the use of invisible inks on paperwork prepared by financial institutions (e.g. checks, account statements, routing documents, and the like). These items typically contain a wide variety of routing codes, numerical identifiers, data summaries, and the like which (for numerous reasons including security-related issues) preferably remain invisible to the unaided eye under the conditions outlined above. Likewise, in many applications, it is desired that "bar-coding" for inventory control, product assembly applications in factories, and other comparable purposes be undertaken in a manner where the particular bar-code of interest does not appear in visible form on the products or documents being processed. Representative patents which discuss the use of invisible ink materials for bar-coding purposes include but are not limited U.S. Pat. No. 5,282,894 to Albert et al.; U.S. Pat. No. 5,348,348 to Hanada et al.; and U.S. Pat. No. 5,686,725 to Maruyama et al. Other patents which generally describe the usefulness of invisible ink materials for a variety of different purposes include U.S. Pat. No. 4,243,694 to Mansukhani; U.S. Pat. No. 4,540,595 to Acitelli et al.; U.S. Pat. No. 5,093,147 to Andrus et al.; U.S. Pat. No. 5,215,838 to Tam et al.; U.S. Pat. No. 5,301,044 to Wright; U.S. Pat. No. 5,423,432 to Krutak et al.; U.S. Pat. No. 5,614,008 to Krutak et al.; U.S. Pat. No. 5,643,356 to Nohr et al.; U.S. Pat. No. 5,684,069 to Auslander; U.S. Pat. No. 5,702,511 to de Saint-Romain et al.; U.S. Pat. No. 5,703,229 to Krutak et al., and others.
In addition to the various patents which exist involving invisible ink materials in general, a number of patents have been granted which describe specific approaches for handling and formulating invisible ink compositions. For example, certain references disclose invisible dye compounds which are "complexed" (e.g. chemically coupled or otherwise joined) with a variety of polymeric materials (see U.S. Pat. No. 5,614,008 cited above). The polymers are apparently designed to increase the fluorescence intensity of the dyes. Notwithstanding the benefits associated with this process, the presence of polymeric materials within the completed ink formulations (particularly those that are complexed with the selected dye compound[s]) can diminish print quality levels and reduce overall printer reliability in applications involving high speed/high resolution inkjet printing. This situation can occur because these polymers often form undesired films or deposits within the printing system which interfere or otherwise prevent effective ink drop formation. As a result, images with poor print quality and inadequate edge acuity are generated.
With continued reference to inkjet technology, this approach is of considerable interest in the marking of substrates using invisible inks. Inkjet printing techniques are characterized by a high degree of operational efficiency, low cost, excellent print quality, and rapid ink delivery. Thermal inkjet printing units are especially important in this regard. Printing systems which employ thermal inkjet technology basically involve a cartridge unit having at least one ink reservoir chamber in fluid communication with a printhead. The printhead includes a substrate (preferably made of silicon) that comprises a plurality of thin-film heating resistors thereon. Selective activation of the resistors causes thermal excitation of the ink materials retained within the ink cartridge and expulsion thereof from the cartridge. Representative thermal inkjet systems are discussed in U.S. Pat. No. 4,500,895 to Buck et al.; U.S. Pat. No. 4,771,295 to Baker et al.; U.S. Pat. No. 5,278,584 to Keefe et al; and the Hewlett-Packard Journal, Vol. 39, No. 4 (August 1988), all of which are incorporated herein by reference. Further information regarding inkjet printing devices (including those which incorporate thermal inkjet technology) will be discussed below relative to the present invention.
The invention claimed herein shall be applicable to all types of inkjet printing systems including those which employ cartridge units having a self-contained supply of ink within a housing that is directly attached to a printhead, as well as alternative inkjet systems which use an ink supply that is remotely positioned from the printhead and in fluid communication therewith using one or more conduit members. The claimed materials and methods are also applicable to inkjet printing units using other (e.g. non-thermal) ink delivery methods including those which incorporate, for example, piezoelectric technology as discussed further below.
Inkjet printing techniques and the use of invisible ink materials for the purposes outlined above (and other related applications) offer many important benefits. In accordance with the specialized components employed in inkjet printing systems (particularly thermal inkjet units) which typically include numerous small openings, passageways, and the like through which ink materials must pass, the inks selected for use in these systems must be carefully considered. Otherwise, print quality deterioration and a decrease in operating efficiency can occur. In addition to these factors, the ink materials of interest must comply with many other requirements including high levels of waterfastness, lightfastness, fluorescence intensity, bleed resistance, and the like. The present invention involves specialized invisible ink compositions which are particularly well-suited for use in inkjet printing systems (especially those which employ thermal inkjet technology). Likewise, the materials and methods described herein overcome numerous problems associated with prior invisible ink formulations and offer many advantages including but not limited to (1) high print quality levels (particularly when thermal inkjet technology is employed); (2) superior lightfastness and waterfastness; (3) excellent fluorescence intensity during illumination with an appropriate light source; and (4) a high level of reliability when used in connection with inkjet printing systems (particularly those which employ thermal inkjet technology).
Accordingly, the present invention represents an advance in the art of invisible ink imaging which satisfies a long-felt need as noted above. It will become readily apparent from the following discussion that the invention is novel in the materials and procedures that it employs, as well as the results which it obtains. The claimed invention therefore constitutes a unique development of considerable significance which will now be discussed in detail.